1. Field of the Invention
The present invention relates to a system for controlling an output power of an internal combustion engine, and more particularly to a system for controlling an output torque of an internal combustion engine having a plurality of cylinders with ignition plugs.
2. Description of the Related Art
In order to decrease the output torque of an internal combustion engine for controlling an engine output power, it has been proposed to place a part of or all of a plurality of engine cylinders in a resting state by cutting-off a supply of a fuel to relevant cylinder or cylinders. In the present specification, this control is called a fuel cut-off (F/C). This type of engine output control will be hereinafter referred to as "split cylinder control". Further, the above mentioned control system is applied to the traction control system (TSC) for improving the fuel consumption and drivability of automobiles.
In this case, to control or adjust the engine output torque in stepwise fashion, the number of the engine cylinders placed in the resting state is controlled.
In Japanese Unexamined Patent publications Nos. Kokai Sho 58-8436 and Kokai Hei 1-130018, an engine output control system is described in which the number of cylinders to be brought into the resting state is previously determined for respective control modes in accordance with an amount of the engine output to be decreased.
Also, it has been proposed to perform the engine output control by adjusting or retarding a spark ignition timing ill combination with the above-mentioned split cylinder control. For example, such a method has been disclosed in Japanese Unexamined Patent Publication Kokai Hei 3-246334.
In this known engine output control system, as shown in a data map of FIG. 23, a pattern for denoting one or more cylinders to be placed in resting state and a retard control of the spark ignition timing is previously formed in accordance with necessary amounts of reduction in the engine output. It should be noted that the data map shown in FIG. 23 is formed for controlling the output torque of a four cylinder engine. In this example, when the spark ignition timing retard control is performed, the engine output torque is reduced by 12.5% of the maximum power, said amount corresponding to a half of a nominal output torque produced by a single cylinder which is fired at a normal ignition timing. Hereinafter, this amount of the reduction in the engine output torque is referred to as torque down magnitude. In FIG. 23, #1, #2, #3 and #4 represent first, second, third and fourth cylinders, respectively which are successively fired in this order in the normal condition without the F/C control and retard control. On the other hand, in F/C command, a circle mark represents an active cylinder which receives a fuel supply and a cross mark represents a resting cylinder due to the fuel cut-off. Also, in a retard command, a mark - represents the normal spark ignition timing and R represents the retarded ignition timing.
It should be noted that the above control is to reduce the output torque of the engine, but a similar control has to be carried out to increase the output torque of the engine during a torque recovery.
When both the split cylinder control and ignition timing retard control are performed in the engine output control, more precise output torque control than that achieved solely by the split cylinder control can be attained. However, the known systems for controlling the output torque of the internal combustion engine have the following problems.
Namely, although by a desired combination of the split cylinder control and the ignition timing retard control, the engine output torque can be decreased or increased in a desired manner during a steady state of the engine, there is a fear that desired engine output torque control cannot be achieved during a transition state of the engine. That is, during the engine transition state in which the number of resting cylinders is changed, desired engine output torque control cannot be performed in certain instances. In such a case, hunting of control might occur and the output torque of the engine might not be decreased or increased into a desired value stably.
The above mentioned problem will be further discussed in terms of the case illustrated in FIG. 24. FIG. 24 shows changes in various parameters such as control mode, cylinders to be fired, fuel supply command, retard command, the number of actually resting cylinders and actual mode when the operating mode is changed from the mode 2 to the mode 1 in accordance with the data map of FIG. 23. During the operation under the mode 2, responsive to a torque-up or torque recovery command, the mode is changed into the mode 1. In the mode 2, the number of cylinders to whom the supply of the fuel is cut-off (F/C cylinder number) is one and the spark ignition timing is not retarded, and in the mode 1, F/C cylinder number is zero, and the spark ignition timing is retarded.
When the torque-up or torque recovery command is given, the number of resting cylinders should be immediately changed to zero. However, in practice, the resting cylinder number does not become zero instantaneously due to a time lag in the operation of the engine. That is to say, the operating condition of the engine could not be changed at a high speed and there is always a time delay. Therefore, even if the command for changing the F/C cylinder number from 1 to 0 is provided, a resting cylinder could not be immediately returned into an active cylinder so that the torque recovery is delayed.
Accordingly, when the ignition timing retard control is performed immediately after varying the mode from the mode 2 to the mode 1, operations of both the mode 2 and mode 1 take place, and one of the cylinders still remains in the resting condition and at the same time the spark ignition timing retards. This operating condition corresponds to the mode 3. Therefore, the output torque cannot be increased as desired and the undesired hunting occurs in the output power of the engine during the transition as illustrated in FIG. 25. Thus, when the mode steps down to increase the engine output torque delivered[to the driving wheels as driving torque, retardation of the spark ignition timing becomes effective at an earlier timing than that at which the split cylinder control becomes effective so that the actual mode becomes equivalent to the step-up mode to cause unexpectedly lowering of the torque.
It should be noted that the above explained problem also occurs when the control mode changes to decrease the output torque of the engine. That is, during the transition in the torque down command, the output torque of the engine is not decreased smoothly, but shows an undesired aberration.
While the engine output control employing the split cylinder control and the spark ignition timing control in combination is able to precisely adapt the engine output torque to a demanded output torque restriction magnitude, the control will become more effective if the above mentioned problems of hunting and undesirable torque variation can be solved.